Modern integrated circuits are formed on the surfaces of semiconductor substrates, which are mostly silicon substrates. Semiconductor devices are isolated from each other by isolation structures formed close to the surface of the respective semiconductor substrates. The isolation structures include field oxides and shallow trench isolation (STI) regions.
Radio frequency current entering from a noise input, for example, antenna is coupled through substrate and the isolation structures. The coupling current in the substrate and isolation structures may cause unwanted resistance and capacitance to an output device, for example, low noise amplifier (LNA) and power amplifier (PA). The coupling current may be attenuated by enlarging the distance (increasing the region of isolation structures) between the noise input and output. However, with the down-scaling of integrated circuits, it is unlikely to yield extra spaces for more isolation structures. The manufacturing cost will increase, and a larger isolation structures may undermine the ability to a higher device speed.